Microwave assisted chemistry refers to those techniques in which the initiation, acceleration, or other enhancement of an intended chemical reaction is encouraged by the application of microwave radiation to the chemical reaction. In many circumstances, where either reactants or the solutions or mixtures in which the reactants are found are susceptible to microwave radiation, the microwave radiation takes the place of conventional heating. For a number of reasons which are well understood and widely explained elsewhere, microwaves interact directly with such materials and thus tend to heat them much more quickly than other heating methods such as radiant conduction or convection heating. As a result, many chemical reactions can be carried out much more quickly in a microwave assisted environment than they can using conventional (e.g., conduction or convection) heating.
In addition to being enhanced by the application of microwaves, certain chemical reactions are preferably carried out under pressure. In many such circumstances, the reagents, solvents or other carriers generate the pressure as they evaporate into gases in a closed reaction vessel under the influence of the microwaves. When such reactions are being carried out, it is often desirable, and sometimes necessary, to monitor the temperature and pressure inside the vessel. Monitoring the temperature and pressure give a useful indication of the progress of certain reactions, can be used with feedback circuits and controllers to moderate the amount of microwave radiation being applied to a reaction, and in some cases, provide a necessary safety factor so that the application of microwaves can be stopped if pressure or temperature reach certain predetermined values.
One preferred method of measuring pressure in a reaction vessel during the application of microwaves is the use of a transducer type of sensor. Used in its broadest sense, the term "transducer" refers to a device which measures a primary signal and converts it into a secondary signal. The secondary signal is then used in a monitoring or control scheme. Pressure is considered to be a mechanical primary signal, although other primary signals can include thermal, electrical, magnetic, radiant, or even chemical signals. Because devices useful in microwave assisted chemistry are often used in conjunction with control circuits that include microprocessors, a preferred secondary signal from a transducer is an electrical signal. It will be recognized, however, that the secondary signal could also be mechanical, thermal, magnetic, radiant, or chemical in nature. For the sake of clarifying the discussions herein, the term "pressure transducer" will be primarily used to refer to a device in which the mechanical pressure exerted by the chemical reaction is translated into an electrical signal.
Because microwaves are electromagnetic radiation, however, they tend to interfere with the operation of electrical devices such as transducers. Alternatively, even if the microwaves don't interfere with the devices themselves, they may interfere with the signals generated by and transmitted from the transducer. Accordingly, in conventional microwave assisted chemical systems, the pressure transducer is typically located outside of the resonator cavity in which the reaction vessels are being exposed to the microwaves. In order to monitor the pressure, an appropriate pressure-resistant hose runs from the vessel, through the wall of the cavity, and then externally to the transducer.
Such arrangements of the vessel inside the cavity, the transducer outside the cavity, and the connecting pressure hose raises particular problems. First, the hose cannot be disconnected from the vessel or the transducer until the pressure in the vessel is otherwise released. Internal pressures in such reaction vessels often are quite high, in some cases 800 pounds per square inch (psi) or more. Theoretically, an in-cavity disconnect coupling of some type could be used a part of the pressure hose. Such a coupling would have to be both microwave-transparent while sufficiently strong to withstand the high pressures. To date, however, such couplings are either unavailable, too inconvenient for reasonable use (e.g., size and positioning problems), or so expensive as to be commercially unreasonable with respect to the overall cost of the device.
Accordingly, in commercial devices, the transducer must be maintained on the outside, and the vessels cannot be disconnected from the transducer until they have cooled sufficiently to reduce the pressure in the vessels to manageable and safe levels. As a result, although microwaves can accelerate certain reactions to completion relatively quickly, the cooling down and depressurization of the vessels can take a disproportionately long time, thus slowing down the overall turnaround rate of the reactions. Because one advantage of microwave assisted chemistry is its enhanced speed, the requirement of waiting for vessels to cool and depressurize moderates some of that advantage. For example, many laboratory microwave devices hold six or more of the high pressure reaction vessels, so that six or more reactions (usually with identical reagents) can be carried out at the same time. These devices can be used repeatedly to run dozens of tests in a relatively short time, except for the down time required for one set of vessels to cool before they can be disconnected and removed from the microwave cavity.
Accordingly, a need exists for an apparatus in which vessels can be removed from the cavity while still hot and under high pressure, but without having to disconnect them from the transducer while the transducer is being exposed to the high pressure from the vessels.